Maybe you’ve noticed that when working on a project (a scarf, blanket, etc). the foundation chain makes the work narrower at the bottom and the following rows just get bigger at the sides and center. 
The trick to make it even is quite simple: Use one hook size bigger for the foundation chain.  
If your project requires – let’s say – a 4mm hook, make the foundation chain with a 5mm hook and then switch to 4 mm in the first row. The work will be perfectly aligned. 

For the mustard yarn above, the first (top) sample is made with a 4 mm hook for both foundation and row 1, while for the sample below it, I used a bigger hook (5 mm ) for the foundation and then the required hook (4 mm) for row 1. I made dc stitches.

With the pink yarn I applied the same tip with sc stitches and the difference is quite striking. The top part tends to make a kind of an arch over the foundation. The first sample is made with just a 3 mm hook, while for the one below it, I used a 4 mm hook for the foundation and switched to a 3 mm hook for row1.

Do you do this with your foundations? Any tips you could share?